1. Field of the Invention
The present invention relates to a system for facilitating data communications between an ISDN subscriber's terminal or a packet terminal in a packet service network and a start-stop terminal connected to an existing telephone network via a modem and an analog line.
2. Description of the Related Art
With the widening use of Integrated Service Digital Networks (ISDNs), requirements for data communications between a terminal connected to an ISDN via a digital line and a terminal connected to an existing telephone network via an analog line have increased. To meet these requirements by the use of existing equipment and communication systems, a modem must be installed on the side of a terminal connected to te ISDN via the digital line.
FIG. 1 illustrates a system for carrying out data communications between a terminal connected to an ISDN via a digital line and a terminal connected to an existing telephone network via an analog line by the use of existing equipment and communication systems.
First, to connect to an ISDN terminal (hereinafter referred to as start-stop transmission terminals) for making start-stop transmissions in an existing telephone network, a Public Telephone Network (PSTN) 102 is connected to an ISDN switching unit 101 via a trunk and start-stop transmission terminals 104-1, 104-2 are connected to the telephone network 102 via modems 103-1, 103-2.
ISDN subscriber terminals 105-1, 105-2 are connected to the ISDN switching unit 101 via network terminators Nt 106-1, 106-2 and terminal adapters TA 107-1, 107-2. For data communications between the ISDN subscriber terminal 105-1 and the start-stop transmission terminal 104-1 connected to the Public Telephone Network PSTN 102 via an analog line, a modem 103-3 must be installed between the ISDN subscriber terminal 105-1 and the terminal adapter TA 107-1.
The ISDN provides packet switching connection as a basic connection service. For the packet switching connection, a packet handler 108 is attached to the ISDN switching unit 101 to carry out protocol translation between the ISDN switching procedure and the packet switching procedure. A packet network 109 is connected to the ISDN via the packet handler 108. A packet terminal 110 is connected to the packet network 109.
Described next is a system for facilitating data communications among various types of terminals interconnected as described above.
First, for existing data communications between the start-stop terminals 104-1 and 104-2, a communication path Pa is established from the start-stop transmission terminal 104-1 through the modem 103-1, the telephone network PSTN 102 and the modem 103-2 to the start-stop transmission terminal 104-2. For data communications between the ISDN subscriber terminals 105-1 and 105-2 within the ISDN, a communication path Pb is established from the ISDN subscriber terminal 105-1 through the terminal adapter TA 107-1, the ISDN switching unit 101, the network terminator NT 106-2 and the terminal adapter TA 107-2 to the ISDN subscriber terminal 105-2.
Moreover, for data communications between the packet terminal 110 and the ISDN subscriber terminal 105-1, a communication path Pc is established from the packet terminal 110 through the packet network 109, the packet handler 108, the ISDN switching unit 101, the network terminator 106-1 and the terminal adapter 107-1 to the ISDN subscriber terminal 105-1.
For data communications between the start-stop transmission terminal 104-1 connected to an analog network and the ISDN subscriber terminal 105-1, the modem 103-3 must be installed between the ISDN subscriber terminal 105-1 and the terminal adapter TA 107-1 . A communication path Pd is thereby established from the start-stop transmission terminal 104-1 through the modem 103-1, the telephone network PSTN 102, the ISDN switching unit 101, the network terminator NT 106-1, the terminal adapter TA 107-1 and the modem 103-3 to the ISDN subscriber terminal 105-1. A modem is needed because the ISDN terminal is not provided with any facility for terminating a protocol for the start-stop transmission.
Thus, the above conventional system has the following problems. First, a modem must be installed on the side of an ISDN subscriber terminal. That is, for an ISDN subscriber to establish communication with a terminal in an analog telephone network, a modem is always needed. This modem must be installed at the expense of the subscriber.
Second, switching between connection and disconnection of the modem installed on the side of the ISDN subscriber terminal is required according to the type of communication. That is, a modem installed at the expense of an ISDN subscriber is required only for communication with start-stop transmission terminals in the analog telephone network and not for communication with other ISDN subscriber terminals and packet terminals. Therefore, the modem must be inserted between the terminal adapter TA and the terminal for communication with a start-stop transmission terminal in the analog telephone network or removed for other communications to thereby directly connect the terminal adapter TA to the terminal.